Serial electrocardiographic changes in necropsy-proven idiopathic dilated cardiomyopathy are evaluated. In addition, a method of predicting heart weight, using QRS amplitudes is described. In 34 patients with multiple electrocardiograms (mean 3/patient) progressive prolongation of PR interval (0.18 plus or minus 0.03 to 0.21 plus or minus 0 .03, p less than 0.001) and QRS duration (0.10 plus or minus 0.02 to 0.13 plus or minus 0.03, p<0.0001) was noted. Progressive conduction abnormalities were common (82%). QTc interval, and QRS and T wave axes did not change. In 50 patients with electrocardiograms within 60 days of death total 12- lead QRS and V1-6 qts amplitude correlated with heart weight (r=0.51, P less than 0.0001 and r=0.55, P<0.0001) better than did the Estes-Romhilt score. The mean total 12-lead QRS amplitude was 138 mm with a mean of 106 for V1-6. In 31 patients cardiac mass index was calculated and showed significant correlation with 12- lead and V1-6 QRS amplitudes (r=0.68. P less than 0.0001 and r=0.75, P less than 0.0001, respectively). The QRS amplitudes remained constant during the illness. By utilizing total 12-lead QRS or frontal plane QRS amplitude heart weight can be predicted as early as 2 years prior to death. Utilization of body surface area and QRS amplitude criteria increases the accuracy of heart weight prediction. Thus we conclude that progressive electrocardiographic changes are common in patients with idiopathic dilated cardiomyopathy and that QRS amplitude criteria are more accurate in the prediction of left ventricular hypertrophy than standard criteria.